Braking system for a motor vehicle

ABSTRACT

A brake system for a motor vehicle is proposed, in which at least the braking force distribution between the front and rear axles is influenced by an automatic electronic controller. In the event of a defect in a rotational speed sensor of a front wheel or a rear wheel, emergency operating measures are proposed, which maintain the functionality of the braking force distribution controller even after a defect has occurred.

STATE OF THE ART

The invention pertains to a brake system for a motor vehicle accordingto the introductory clause of claim 1.

A brake system of this type is known from DE 4 12 388 A1 (U.S. Pat. No.5,281,012). The brake system described in that document has anelectronic control unit, which at least adjusts the braking forcedistribution between a front axle and a rear axle. The braking forcedistribution between the front axle and the rear axle should be as closeas possible to the ideal braking force distribution with respect todeceleration and stability of the vehicle. This is achieved by means ofan automatic controller, in which the difference between the slowestrear wheel and the fastest front wheel is compared with a predetermineddifference value; the braking pressure is then adjusted in the rear axlebrakes in such a way that the predetermined value is at least notexceeded. The electronic controller limits the pressure in the rear axleand prevents the rear axle wheels from locking before the front axlewheels. In the known automatic controller, it is assumed that therotational speeds of the wheels of the vehicle are determined correctly.No measures for dealing with the occurrence of a defect are presented.

It is therefore the task of the invention to provide measures whichensure that the braking force distribution controller remains functionaland that the rear wheels will not lock before the front wheels do evenwhen there is a defect in one of the rotational speed sensors.

This is achieved by means of the characterizing features of claim 1.

Many different measures for testing the functionality of rotationalspeed sensors or for detecting defects in such sensors are known (see,for example, DE 34 18 235 C2).

ADVANTAGES OF THE INVENTION

By means of the procedure according to the invention, the advantageouseffects of an electronic braking force distribution controller areobtained even when one of the rotational speed sensors fails.

It is especially advantageous that the advantageous effects are obtainedwhen a rotational speed sensor at a front wheel fails and also when arotational speed sensor at a rear wheel fails.

The functionality of the braking force distribution controller ismaintained advantageously in such cases, and as a result the rear wheelsare prevented from locking before the front wheels during a brakingprocess. It is particularly advantageous, furthermore, that thestability of the vehicle is improved in vehicles with an overbraked rearaxle.

By means of supplemental measures, the stability of the vehicle isimproved when it is braked while traveling around a curve both in theevent of the failure of a rotational speed sensor on a front wheel andalso in the event of the failure of a rotational speed sensor on a rearwheel.

Additional advantages can be derived from the following description ofexemplary embodiments and from the dependent claims.

DRAWING

The invention is explained in greater detail below on the basis of theembodiments illustrated in the drawing.

FIG. 1 shows an overall block circuit diagram of a brake system withelectronic braking force distribution control, in which the measuresaccording to the invention are used in the event of a defect.

FIGS. 2 and 3 show flow charts, which sketch the procedure according tothe invention which is implemented when a rotational speed sensor failsat a front wheel and at a rear wheel, respectively.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows an electronic control unit 10, which consists of at leastone microcomputer (not shown). With reference to the control of thebraking force distribution between the front and rear axles, controlunit 10 or the microcomputer here comprises essentially an appropriateautomatic controller 12. Output line 14 of control unit 10, i,.e., ofautomatic controller 12, leads to valves 16, which affect the brakingpressure in the rear wheel brakes. Over a line 18, a measure of thedeviation between the nominal and the actual values of the differencebetween the fastest front wheel and the slowest rear wheel istransmitted to controller 12 from a comparison point 20. Over line 22,the nominal value is transmitted to comparison point 20 from a memoryunit 24, and over line 26 the actual value is transmitted from acomparison point 28. So that the difference between the fastest frontwheel and the slowest rear wheel can be found, a line 30 from a maximumvalue selection stage 32 and a line 34 from a minimum value selectionstage 36 lead to comparison point 28. In the preferred exemplaryembodiment of a two-axle vehicle, input lines 38, 40 lead fromrotational speed sensors 42, 44, which detect the rotational speeds ofthe front wheels, to maximum value selection stage 32. Lines 46, 48 leadto minimum value selection stage 36 from rotational speed sensors 50,52, which detect the rotational speeds of the rear wheels. In addition,a defect determination unit 54 is provided, to which lines 56, 58, 60,62 from lines 38, 40, 48, 46 are connected. A first output line 64 ofdefect determination unit 54 leads to maximum value selection stage 32,whereas a second output line 66 leads to minimum value selection stage36.

In the case of a multi-axle vehicle, the layout will be analogous tothat described above for a two-axle vehicle.

In a preferred embodiment, a line 68 leads to memory element 24 fromoutput lines 64, 66. In addition, in a preferred exemplary embodiment, aline 70 leads from line 46 and a line 72 leads from line 48 by way ofswitching elements 74, 76, respectively, to maximum value selectionstage 32. Switching elements 74, 76 are actuated by way of line 78,which proceeds from output line 64 of defect determination unit 54.

In another preferred exemplary embodiment, furthermore, a switchingelement 82 is provided, which is inserted in line 26 between comparisonpoints 20 and 28. This switching element cuts the connection between thecomparison points and connects comparison point 20 to a line 84. Thisline proceeds from a threshold stage 86, to which a line 88 from acomparison point 90 is connected. A line 92 leads to comparison point 90from line 56, and a line 94 leads to it from line 58. Switching element82 is actuated by way of line 96. This is an output line of a logicalAND gate 98, to which a line 99, starting from output line 66 of thedefect determination unit, a line 97, proceeding from a brake lightswitch 95, and a line 93 branching off from line 84, are connected.

The elements shown are used for the control of the braking forcedistribution between the front and rear axles under normal drivingconditions and in the event of the failure of a rotational speed sensorat a front and/or rear wheel. In addition, control unit 10 alsocomprises elements (not shown) for anti-lock protection, for drive slipcontrol, for engine drag torque control, etc. The hydraulic part of thebrake system, which is shown symbolically in FIG. 1 by valve 16,corresponds in the preferred exemplary embodiment to that described inthe above-cited state of the art of DE 41 12 388 A1 (U.S. Pat. No.5,281,012). The procedure according to the invention can be usedadvantageously not only in this type of hydraulic brake system but alsoin hydraulic systems of other designs as well as in pneumatic brakesystems and in brake systems in which the brakes are applied by electricmotors.

The measures shown in FIG. 1 increase the availability of the electronicbraking force distribution control and make it possible to takeadvantage of this automatic control even when one of the speed sensorson a front and/or rear wheel fails. It is thus possible to omithydraulic or pneumatic pressure reducers on the rear axle; and, becauseof the change in the distribution of the braking force, it is possibleto achieve a more uniform thermal load in all the wheel brakes in thepartial braking range even when a defect occurs. The rear wheels areeffectively prevented from locking before the front wheels, and thus thestability of the vehicle is largely guaranteed even in the presence of adefect.

During operation in the absence of any defects, the rotational speeds ofthe front wheels, which are detected by speed sensors 42, 44, are sentto maximum value selection stage 32. There the maximum value of the twomeasurement values is determined; that is, the fastest front wheel isdetermined. At the same time, the rotational speeds of the rear wheels,which are detected by speed sensors 50, 52, are sent to minimum valueselection stage 36. There the minimum value of the supplied measurementvalues is found; that is, the slowest rear wheel is determined. Atcomparison point 28, the difference between the maximum and the minimumvalue is found, and then the deviation between the actual valuedetermined at comparison point 28 and a predetermined nominal value forthe rotational speed difference, which is stored in memory element 24,is calculated at comparison point 20. As a function of the deviationsupplied to it, automatic controller 12, which, in the preferredexemplary embodiment, is designed in accordance with the state of theart cited above, generates output signals for adjusting the rear axlebrake pressure so as to bring the actual value into conformity with thenominal value.

In a preferred exemplary embodiment, the actual value is prevented fromexceeding the predetermined nominal value by the limitation or reductionof the rear axle brake pressure. In other advantageous embodiments ofthe controller, the actual value is adjusted to match the nominal valueby increases and decreases in the rear axle brake pressure.

Over the corresponding lines, the measurement values for the rotationalspeeds of the wheels are also sent to the defect determination unit. Thedefect determination unit evaluates the rotational speed signals of thewheels individually for the purpose of detecting possible defectivestates. If a defect is identified in a front wheel speed sensor, defectdetermination unit 54 transmits a corresponding signal over line 64. Inthe same way, it transmits a corresponding signal over line 66 if itidentifies a defect in one of the rear wheel speed sensors.

If a defect occurs, the valve relay, over which the inlet and outletvalves of the wheel brakes are supplied with voltage, and the actuationof the return pump remain operative. In addition, the defective signalis excluded from the formation of the estimated vehicle velocity, which,under normal driving conditions, is formed on the basis of therotational speed signals from all the wheels. In the case of defect in afront wheel speed sensor, only one front wheel speed signal is availablefor the formation of the reference value and the deviation valuerequired for the braking force distribution controller. The defectivewheel speed signal is also excluded from the maximum value selection, inthat it is set equal to, for example, a minimum value. The defectivewheel speed signal thus no longer has any effect on the referenceformation or on the formation of the deviation for the braking forcedistribution. The correct front wheel speed signal is always assumed inthis case to be the signal from the fastes front wheel. From this pointon, the electronic braking force distribution controller functions inthe same way as it does under normal driving conditions. When there is adefect in a front wheel speed sensor, furthermore, the difference isalso formed between the fastest and the slowest rear wheel, upon whichvalue the automatic control is then based. It is especially advantageousthat, in the event of a defect, the sensitivity of the control isincreased. This means that, in the event of a defect in a front wheelspeed sensor, the controller responds even when the difference issmaller than that which occurs under normal driving conditions. For thispurpose, the nominal value stored in memory element 24 is, in the eventof a defect, switched to a smaller value. When braking on a curve, thedegree of stability can differ depending on the location of thedefective speed sensor (i.e., depending on whether the defective speedsensor is on the wheel on the inside of the curve or on the wheel on theoutside of the curve); the stability can also differ on roads where thecoefficients of friction on one side are different from those on theother side, depending on how these coefficients of friction are related.It is true that these differences in stability can impair the brakingprocess. Nevertheless, the advantages that lateral guidance continues tobe provided and that the rear wheels continue to be prevented fromlocking more than make up for this disadvantage.

Another improvement in stability in the event of a defective front wheelspeed sensor is derived from the following supplemental measure. Theslip calculation for the braking force distribution controller, that is,the formation of the difference between the fastest front wheel and theslowest rear wheel, is changed. For this purpose, in the event of adefect, the difference is calculated between the fastest wheel of all oran auxiliary reference resulting from the fastest of all the remainingwheels and the slowest rear wheel. This is sketched in FIG. 1 in that,in the event of a defect, the rotational speeds of rear wheel speedsensors 50, 52 are also sent to maximum value selection stage 32. Thismeasures improves the stability during braking on a curve when thedefective front wheel speed sensor is on the front wheel on the outsideof the curve. Then the rear wheel on the out side of the curve can bethe fastest wheel. The braking force distribution controller thusresponds by holding or reducing the pressure at the rear axle at anearlier point, which improves the stability of braking on a curve.

In a corresponding manner, the braking force distribution control alsoremains operational in the event of a defect in a rear wheel speedsensor. In this case at least the locking of one of the rear wheels isavoided during a braking operation, and thus the stability of thebraking process is essentially preserved. Here again, the defectivespeed sensor provides no information concerning the rotational speed ofthe wheel. Therefore, in the event of a defect in a rear wheel speedsensor, this speed signal is excluded from the reference and slipcalculation.

The stability when braking on a curve can be improved by the use of thefollowing advantageous measure. In addition to the control of the rearaxle braking pressure on the basis between the fastest front wheel andthe slowest rear wheel, it is provided, in the event of defect in a rearwheel speed sensor, that the braking force distribution function is alsoactivated when a front wheel is slip ping in comparison with the otherfront wheel or in comparison with the reference velocity, that is, whenthe difference between the wheel speeds exceeds a predetermined value.This activation occurs even if little or no slip can be determined inthe remaining rear wheel. Braking force distribution controller 12 isthus activated when a defect signal is received (line 99), when thebrake pedal is actuated (brake light switch 95), and when the speeddifference between the two front wheels exceeds the predeterminedthreshold value (line 93). This measure leads to improved stability whenbraking on a curve when the defective rear wheel speed sensor is on theinside of the curve.

In the simplest form of realization, controller 12 holds the rear axlebraking pressure constant in this operating situation, as long as thedifference between the front wheel speeds exceeds the threshold valuespecified in threshold stage 86. In another advantageous exemplaryembodiment (see FIG. 1), the difference between the front wheel speedsis used as the actual value in place of the difference between thefastest front wheel and the slowest rear wheel, so that, as long as thedifference between the front wheel speeds persists, the rear axlebraking pressure is kept constant or is reduced.

In correspondence with the measure taken in the case of a front wheelspeed sensor, the braking force distribution controller is also designedwith greater sensitivity in the event of a defect of a rear wheel speedsensor in that the experimentally determined nominal difference value isrevised to a smaller value.

The measures described above are sketched in the form of flow charts inFIGS. 2 and 3. These flow charts represent the implementation of theprocedure according to the invention as a computer program. Thesubprogram shown in FIG. 2 is started when a defect occurs in a speedsensor of a front wheel; the subprogram shown in FIG. 3 is started whena defect occurs in a speed sensor of a rear wheel.

As soon as the subprogram illustrated in FIG. 2 starts, the two wheelspeeds of the rear wheel speed sensors V_(VA1) and V_(HA2) and therotational speed sensor signal of the functional front wheel speedsensor (here, for example, V_(VA1) ; V_(VA2) is assumed to be defective)are accepted as input in the first step 100. In the following step 102,the front wheel speed signal of the speed sensor V_(VA2), which has beenidentified as defective, is set to a mini mum value V_(min). In the nextstep 104, the maximum value V_(VAmax) is determined on the basis of thetwo front wheel speed signals, and the minimum value V_(HAmin) isdetermined on the basis of the two rear wheel speed signals. Asmentioned above, in another advantageous exemplary embodiment, all ofthe wheel speed signals, especially the functional wheel speed signals,are used to form the maximum value.

In the following step 106, the difference between the maximum value andthe minimum value is formed. Then, in an advantageous exemplaryembodiment, the nominal difference value Δ_(S) to be used in the case ofa defect is read out in question step 108, and in the following step 110the rear axle braking pressure is adjusted as a function of thedifference between the nominal and actual difference value. After step110, the subprogram terminates and is repeated at the specified time.

After the subprogram according to FIG. 3 has started in the presence ofa defect in a rear wheel speed sensor, the front wheel speeds V_(VA1)and V_(VA2) and the correct rear wheel speed signal (here, V_(HA1) ;V_(HA2) is assumed to be defective) are accepted as input in the firststep 300. In addition, in a preferred exemplary embodiment, the signalstatus BLS of the brake pedal switch is read in as input. Then, in step302, the reference speed signal V_(ref) formed from the correct signalvalues or a signal value derived from it is taken as the second rearwheel speed signal V_(HA2). As an alternative, this speed signal can beequated to a driving speed signal determined in some other way or to amaximum value. In the following question step 304, the absolute value ofthe difference between the speeds of the two front wheels is comparedwith a predetermined threshold value A. If the absolute value of thedifference exceeds this threshold value and simultaneously the brakelight switch signalizes a brake actuation, in step 306 the differencevalue is set to the absolute value of the difference between the speedsof the two front wheels. Then, in step 308, the nominal difference valuepossibly under consideration of the defect situation and the fact thatthe slip between the two front wheels is available as an actual value,is read out; and in the next step 310, the rear axle brake pressure isadjusted as a function of the difference between the nominal differenceand the actual difference. After step 310, the subprogram terminates andis repeated at the specified time.

If the program found in step 304 that the difference between the twofront wheels does not exceed threshold value A, then in step 312 themaximum and minimum values are formed from the front wheel speed signalsand the rear wheel speed signals. In the following step 304, the actualvalue is then calculated from the difference between the maximum andminimum values, which is then adjusted in accordance with steps 308 and310.

The implementation of step 306 has been found to be suitable in anexemplary embodiment. In other exemplary embodiments, the emergencyprocedure in the event of a defect in a rear wheel speed sensor iscarried out only on the basis of steps 312 and 314.

In the case of a two-axle vehicle, the procedure according to theinvention pertains only to individual defects on one axle. If doubledefects occur, that is, if both speed sensors fail, other measures mustbe taken.

If defects are recognized at both a front wheel and a rear wheel, themeasures according to the invention are implemented in combination(without step 306).

The procedure according to the invention is applicable in anadvantageous manner to all braking force distribution controllers whichoperate on the basis of signals representing the rotational speeds ofthe wheels.

We claim:
 1. A brake system for a motor vehicle having front and rearaxles each supporting at least one wheel, said system comprising:anautomatic braking force distribution controller controlling thedistribution of braking force between the front and the rear axles onthe basis of a difference value based on a rotational speed signal ofthe front and on a rotational speed signal of the rear wheels inaccordance with a nominal difference value, a defect determination unitdetermining defects involving the rotational speed signals of the wheelsof the motor vehicle, the automatic braking force distributioncontroller, in the event of a determination of a defect in a rotationalspeed signal of a front wheel and/or of a rear wheel, operating withinthe scope of an emergency operating procedure under exclusion of thedefective initiating an emergency operating procedure under exclusion ofthe defective defective speed signal, in the emergency operatingprocedure the automatic braking force distribution controller controlsthe distribution of braking force between the front and the rear axleson the basis of a difference value based on a non-defective rotationalspeed signal of the front and on a non-defective rotational speed signalof the rear wheels in accordance with the nominal difference, with thesensitivity of the automatic braking force distribution controllerincreased by adjustment of said nominal difference to a smaller value.2. A brake system according to claim 1, wherein, to control the brakingforce distribution between the front and rear axles, a differencebetween the fastest front wheel and the slowest rear wheel is formed andcompared with a predetermined nominal value for the adjustment of therear axle braking pressure.
 3. A system according to claim 2, wherein inthe event of a defect in a rear wheel speed signal, the differencebetween the rotational speeds of the two front wheels is used as theactual difference value.
 4. A system according to claim 1, wherein inthe event of a defect in one of the front wheel speed signals, the speedsignal of the other front wheel is identified as the fastest frontwheel.
 5. A system according to claim 1 wherein the speed signals aregenerated by sensors associated with the wheels, and, in the event of adefective front wheel speed sensor, the fastest of all the other wheelsis used for automatic control.
 6. A system according to claim 1, whereinthe speed signals are derived from sensors associated with the wheels,and in the event of a defect in a rear wheel rotational speed sensor, adriving speed reference signal or a driving speed signal is taken as asubstitute for the defective speed signal.
 7. A system according toclaim 1, wherein in the event of a defect in a rear wheel speed signal,the electronic braking force distribution controller is activated whenthe difference between the rotational speeds of the front wheels exceedsa threshold value during the braking process.